1. Field of the Invention
The present invention relates to a pressure detecting device, and in particular to a pressure detecting device for measuring the pressure of a high temperature fluid, for example, suitable for measuring the combustion pressure in an internal combustion engine.
2. Description of the Related Art
As the general structure of a conventional pressure detecting device, there is known one which comprises a cylindrical housing secured on a pressure chamber wall; a metallic sensing unit which is disposed at an opening at one end of the housing so as to face the inside of the pressure chamber, and a pressure receiver having a strain gauge element disposed in the sensing unit; an output signal being obtained responsive to deformation caused in the pressure receiver.
FIG. 1 is an illustration of an example of a conventional pressure detecting device. The example shows a cylindrical housing 3' which is secured in a pressure chamber wall using a threaded portion 3a' at the outer circumference of a smaller diameter part of the housing. A sensing body 2' provided with an opening 2a' in its side wall is held air-tightly at the front opening of the housing 3'. A diaphragm formed of a ceramic plate is disposed on the opening 2a' for receiving pressure, and a semiconductor strain gauge element (not shown) is secured to the inside surface of the diaphragm 1'.
The strain gauge element is connected to electrodes of a terminal plate 9 through wire 4. An output signal of the strain gauge element is taken out to the outside via the terminal plate 9', a lead pin 91', and lead wire 6'.
In the above conventional pressure detecting device, however, even the wire 4', terminal plate 9', lead pin 91', and other parts which take out the output signal from the strain gauge element are inserted into the high temperature pressure chamber. In particular, the bonding assembly including the wire 4' between the diaphragm 1' and the terminal plate 9' is exposed to high temperatures. Accordingly, reliability of the bonding assembly cannot be ensured.
On the other hand, in an apparatus of this kind, the sensing unit is constituted by a so-called SOI structure of semiconductor strain gauge elements made of monocrystalline or polycrystalline silicon formed on an insulative substrate. Since this kind of apparatus does not have a PN junction as do diffusion type gauges, there is no leakage current at a high temperatures and it is possible to detect pressure at high temperature environment of more than 200.degree. C.
As a sensing unit with an SOI structure, there is known one which uses monocrystalline silicon as the insulative substrate, on which semiconductor strain gauge elements made from monocrystalline or polycrystalline silicon are formed through insulative layers.
However, in the conventional art, metal is used for the housings, etc., of high temperature pressure detecting devices. For this reason, in a pressure detecting device using a silicon substrate as an insulative substrate there has been the problem that it is impossible to achieve a high sealing properly in bonding the silicon substrate and the metallic housing unless use is made of glass bonding, which is not that strong in bonding strength.